#define _CRT_SECURE_NO_WARNINGS 1
class Solution {
public:
    int uniquePaths(int m, int n)
    {
        vector<int> dp1(n, 1);
        vector<int> dp2(n, 1);
        for (int i = 1;i < m;++i)
        {
            for (int j = 1;j < n;++j)
            {
                dp2[j] = dp2[j - 1] + dp1[j];
            }
            swap(dp1, dp2);
        }
        return dp1.back();
    }
};